The present invention relates generally to Magnetic Resonance Imaging (MRI) systems, and more particularly, to an embedded thermal control system for high field MR scanners.
Magnetic Resonance Imaging (MRI) is a well-known procedure for obtaining detailed, two and three-dimensional images of patients based on nuclear magnetic resonance (NMR) principles. MRI is well suited for the imaging of soft tissues and is primarily used for diagnosing internal injuries.
Typical MRI systems include a magnet capable of producing an intense, homogenous magnetic field around a patient or portion of the patient; a radio frequency (RF) transmitter and receiver system, including a receiver coil also surrounds a portion of the patient; a magnetic gradient system localizes a portion of the patient; and a computer processing/imaging system, which receives the signals from the receiver coil and processes the signals into interpretable data, such as visual images.
The superconducting magnet is used in conjunction with a magnetic gradient coil assembly, which is sequentially pulsed to create a sequence of controlled gradients in the main magnetic field during a MRI data gathering sequence. The superconducting magnet and the magnetic gradient coil assembly include the radio frequency (RF) coil on an inner circumferential side of the magnetic gradient coil assembly. The controlled sequential gradients are effectuated throughout a patient imaging volume (patient bore) which is coupled to at least one MRI (RF) coil or antennae. The RF coils and a RF shield are typically located between the magnetic gradient coil assembly and the patient bore.
As a part of a typical MRI, RF signals of suitable frequencies are transmitted into the patient bore. Nuclear magnetic resonance (NMR) responsive RF signals are received from the patient via the RF coils. Information encoded within the frequency and phase parameters of the received RF signals, by the use of a RF circuit, is processed to form visual images. These visual images represent the distribution of NMR nuclei within a cross-section or volume of the patient within the patient bore.
In modem MRI, active electric coils are used to drive spatial gradients into the static magnetic field. Enhanced imaging sequences typically demand high amplitude gradient fields, rapid field transitions, and large duty cycles in order to improve resolution and scan time. Unfortunately, these properties also drive the power dissipation higher and thus cause higher temperatures in the scanner. For many desired use profiles, the resulting scanner temperatures would exceed the allowable limits and would thus force a halt in operation while the scanner cooled down. Historically, this halting has been avoided by setting a constant limit on a basic quantity, e.g. coil current. While this limits the peak power in a coil, it is independent of temporal response and therefore employs assumptions concerning the use profile and boundary conditions. Those assumptions are generally conservative so as to limit any risk of patient exposure to excessive temperatures. Thus, the historic scheme for limiting power into gradient coils often places unnecessary limits on the gradient fields available to the prescriptions because the actual use timelines and boundary conditions are ignored.
It would therefore be desirable to include an enhanced imaging sequence in an MRI without placing unnecessary limits on gradient fields. It would also be desirable to limit peak power in a coil in response to actual use timelines. The present invention is directed to these ends.